Method of making a heated extended resistance temperature sensor

ABSTRACT

An extended resistance temperature sensor formed of a plurality of lengths of pre-insulated resistance temperature detector (RTD) wire. The RTD wire is either unheated, self heated or is heated by means of strands of heater wire integrated with or placed closely adjacent to the RTD wire. The RTD wire itself, or together with the heater wire, is bonded together in one embodiment. Alternatively, the RTD wire, or with the heater wire, may be encased in insulated shrink tubing, thin wall metal tubing or both. Connectors are provided to supply electrical current for heating purposes or to connect the RTD wire to detection circuitry, or both. Protective sheaths are provided over the connectors.

This is a divisional of copending application Ser. No. 07/189,387 filedon May 2, 1988 now U.S. Pat. No. 4,994,780.

FIELD OF THE INVENTION

This invention relates generally to electrical resistance temperaturesensors and more particularly to a long, slender sensor capable ofsensing continuously over an extended field, the sensing element of thesensor being continuous, insulated resistance temperature detector (RTD)wire formed in a bundle of parallel strands.

BACKGROUND OF THE INVENTION

Both thermocouples and RTD's are in widespread use for sensingtemperature and providing an electrical output representative of thetemperature sensed. Thermocouples, by their nature, are point sensorsbecause they thermoelectrically produce an electromotive force at aspecific junction between two different materials. RTD's employ a wiresensing element which has a resistance which varies with temperature.Most RTD's are now designed to concentrate the electrical resistance toa small point or in the smallest possible volume, with miniaturizationbeing a principle feature so that RTD's are, like thermocouples,essentially point sensors. Because of this point sensing feature,whenever an extended field is to be interrogated with the use of eitherthermocouples or RTD's, it has generally been necessary to distribute amultiplicity of differentially heated differential thermocouples ordifferential RTD's and measuring the heat loss at the particular pointwhere the differential temperature is being sensed.

No matter how many point sensing thermocouples or RTD's are distributedin the field, they are unable to provide an accurate, true analogrepresentation of the information to be determined from the fieldbecause they are still only sensing specific points. Determining thebest points to interrogate, installing the individual sensor elements,and making the numerous required individual electrical connections tothe point sensing elements in accordance with the generally acceptedtechnology, are cumbersome and expensive steps.

There are many situations where it is desirable to sense level or flowrate over an extended field. This has been accomplished to a certaindegree with thermocouples and RTD's by converting the point sensingreading to an average temperature of the field. However, the larger sucha differential thermal field is, and the more varied the temperaturesare across the field, the more point sensing elements are required toobtain a readout which is reasonably representative of the averagecondition of the field.

One situation where extended field interrogation is currently made withthermocouples or RTD's involves gauging of the fluid level, or thelocation of a phase change interface such as between liquid and gas, ina vessel such as a tank. This type of level gauging can currently beaccomplished with thermocouples and RTD's by arranging a series ofspaced sensor elements along the height of the tank, that is, atvertically separated points in the field being interrogated. In the caseof RTD's, a series of heated RTD's and companion reference RTD's areemployed along the height of the tank. As liquid reaches each RTD pointsensor, the sensor reports that it is wet when it is cooled by thehigher thermal dispersion rate of the liquid than is the case for theair or gas phase above the surface of the liquid. However, the operatoris unable, with such point sensing structure, to determine whether theliquid level is just at that particular point or at any level betweenthat point and just below the next higher RTD sensing point. Furtherfilling of the tank will result in discrete reports from thesequentially higher RTD's, while lowering of the liquid level will causesuccessive discrete reports from the successively lower RTD's as theyare uncovered from the liquid. For example, if ten sensing points areemployed along the height of the tank, each with an individual heatedRTD sensor and a reference RTD sensor, the gauging can only be performedat ten individual step points with unresolvable uncertainty of where theliquid level is between any two of those points. The only way to reducesuch uncertainty when employing point sensors is to increase the numberof sensing elements, at correspondingly increased expense, cumbersomewire connections and possibly reduced reliability.

Accurate liquid level sensing is of critical importance in any liquidstorage vessels and particularly in reactor buildings of nuclear powerplants, as well as in the reactor vessels themselves. This accurateliquid level sensing is important in avoiding nuclear power plantaccidents which could be caused when the actual level of the liquid iseither not properly known or is misinterpreted. In addition to lack ofdesired accuracy, liquid level changes are not immediately sensed whenpoint sensors are used since there can be considerable change in liquidlevel prior to detection by the next sensing element which is eithercovered or uncovered. Thus, a developing problem or trend may not beimmediately detected and the desirable mitigating action to suppress orcorrect the problem cannot be taken in as timely a manner as may bedesirable or necessary.

Each of the vertical sequence of thermocouples or RTD's in such a liquidlevel gauging system requires its own separate electrical connections tothe detection circuitry. The thereby required large number of joints orsplices can result in undesirably low reliability, which could beespecially dangerous in the environment of a nuclear power plant. As anexample of this problem, there is in existence a point-sensing RTDsystem for water level sensing in a nuclear reactor building which hasapproximately fifty RTD sensors arrayed over a vertical height of aboutsixty feet.

RTD's are preferred for some purposes over thermocouples because theycan be made more sensitive, being able to provide an output signal manytimes greater than is generally possible with thermocouples. This isprimarily because RTD's operate with an external electrical power sourcewhich can provide as high a level of voltage or current as is desired.Thermocouples operate on the basis of a self-generated junctionelectromotive force (emf) which inherently has a very low output voltagelevel as well as other inaccuracies.

For sensing in some extended fields, such as the inside of a nuclearreactor vessel, access may be relatively difficult and may be bestachieved by encasing a series of sensors in a long, slender, tubularprobe. Such a probe can be readily inserted in an existing reactorvessel instrument guide tube. RTD's are desirable in such situationsbecause of their high output and therefore high sensitivity but manyprior art RTD's are not suitable for such packaging, being too bulky andhaving a ceramic or a glass insulator too brittle to allow them to bedeformed as would be required for packaging in a long, slender, tubularprobe.

On the other hand, thermocouples have been packaged inside a metalcasing as small as 0.01 inch in diameter. A series of such encasedthermocouples and the required electrical leads may be placed inside atube and encased by drawing or swaging the tube down around thethermocouples and leads to produce a long slender probe suitable forgaining access in restricted regions inside a nuclear reactor vessel.This advantage for the thermocouples is balanced by at least oneequivalent disadvantage. Thermocouples are relatively delicate and areeasily subject to breakage during the manufacture of such probes or uponaccidental impacting. Of course, as discussed above, a thermocouplebeing a point sensor, thermocouple probes necessarily have a stepfunction output rather than a continuous output, so the accuracy ofliquid level determination is limited. Additionally, the electricaloutput of thermocouples is so small that performance is grainy andresolution and accuracy are relatively poor. Because individual wireleads are required for each thermocouple, numerous wires must extendalong the probe, thereby limiting how small the outside diameter of atubular probe can be. Of course, the greater the number of thermocouplesplaced along the probe in any attempt to increase resolution the greaterthe number of leads. This large number of leads also seriously reducesthe reliability of thermocouple-type probes. Thermocouple probes arerelatively expensive to make, especially considering the number ofleads, connections and electronic cooperating devices required for suchprobes.

Another example of an extended field which has been interrogated by amultiplicity of RTD's or thermocouple sensors, is a large duct having anon-uniform flow profile, where it is sought to obtain an averagereading of the flow velocity in the duct. Such non-uniform flowdistributions exist, for example, in air ducts where diameters are largeand fittings such as tees, elbows, transitions, bends, section changes,louvers, dampers and the like cause flow disturbances. Non-uniform flowdistributions also typically occur in the input air ducts and combustionoutput ducts of fossil fuel power plants. In such cases, a multiplicityof point sensing elements is placed at what are considered to bestrategic locations across the gas flow path, but only a roughapproximation of the flowable velocity can be obtained by the use ofsuch discrete, point sensing locations. As stated previously, a largenumber of individual point sensors results in high costs due to thenumber of leads, connectors and mating electronic devices that arenecessary to cooperate in interpreting the individual signals.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an elongated sensor formedof RTD wire suitable for interrogating an extended field. Broadlyspeaking, the sensor of this invention comprises a plurality of strandsof pre-insulated RTD wire arranged in a series of interconnected loopsand having means to bundle or encase the strands in a protective sheathand means to connect the RTD wire to detector circuitry and selectivelyto electrical current for self heating purposes. An alternativeembodiment employs a separate pre-insulated heating wire also having oneor more interconnected strands and arranged closely adjacent to orintermingled with the insulated RTD wire. In that case the electricalcurrent would be applied to the heating wire for generating the desiredheat.

This invention is an improvement over that shown in applicant's Pat No.4,977,385 for another type of distributed RTD sensor. Because the mannerin which this sensor is used is equivalent to that of the earliersensor, reference to the other application will be made several timesherein below

The extended resistance temperature sensor may take the form of thestrands of pre-insulated sensor wire bonded together to form asemi-rigid elongated sensor or the strands of pre-insulated sensor wiremay be encased in shrink tubing or small diameter thin wall metaltubing. In another alternative embodiment, either the sensor or theheater wires are in elongated form and the heater or sensor wires arewrapped around the elongated wires in spiral fashion. This embodimentmay either be enclosed in shrink tubing, thin wall metal tubing orwithout any protective sleeve or tubing.

BRIEF DESCRIPTION OF THE DRAWING

The objects, advantages and features of this invention will be morereadily perceived from the following detailed description when read inconjunction with the accompanying drawings, in which:

FIG. 1 shows a sensor of this invention in its basic form;

FIG. 2 is a cross section through cutting plane 2-2 of FIG. 1;

FIG. 3 shows a sensor of the invention with pre-insulated sensor wireand pre-insulated heater wire shown in spaced relationship forexpository purposes;

FIG. 4 is a cross section showing how the heater and sensor wires ofFIG. 3 may be randomly intermingled in a preferred embodiment;

FIG. 5 is a view similar to FIG. 1 showing the pre-insulated sensor wireencased in small diameter thin wall metal tubing prior to reducing thetube diameter;

FIG. 6 shows the sensor wire combination of FIG. 3 encased in plasticshrink tubing but with the sensor and heater wires shown somewhat spacedto aid in depiction and prior to shrinking the tubing;

FIG. 7 is an alternative embodiment with one type of pre-insulated wirein elongated multi-strand form and the other type of pre-insulated wire,either sensor or heater wire, being wrapped around it in spiral fashion;

FIG. 8 is a cross section through cutting plane 8--8 of FIG. 7;

FIG. 9 is an example of the electrical circuitry with which the sensorsof FIGS. 1-8 may be employed;

FIG. 10 is a simplified fragmentary vertical section illustrating a tankwith a matched, parallel pair of distributed RTD's of the inventionvertically deployed along a wall of the tank for liquid level gauging;

FIG. 11 is a side elevational view, with a portion broken away,illustrating a duct elbow with a matched pair of distributed RTD's ofthe invention deployed across it;

FIG. 12 is a simplified transverse sectional view taken on cutting plane12--12 in FIG. 11, with the diameters of the distributed RTD'sexaggerated relative to their lengths for illustrative purposes;

FIG. 13 is a simplified transverse sectional view similar to FIG. 12showing the distributed RTD's of this invention arranged to provide avariable function response corresponding to the variable diametricalarea of the fluid flow in the duct elbow; and

FIG. 14 is a sectional view similar to FIG. 13 with the distributedRTD's located in a straight section of the duct.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawing and more particularly to FIGS. 1 and 2thereof, there is shown a sensor constructed in accordance with theinvention. The sensor is formed of a multiplicity of strands ofpre-insulated RTD wire 11. Ends 12 and 13 of the sensor wire areconnected by appropriate means such as welding or soldering to strandedlead wires 14 and 15, respectively. For physical and electricalprotection, a short piece of small insulator shrink tubing 16 and 17,respectively, is secured tightly over each connection of the sensor wireand the stranded lead wire. In this embodiment, another piece of shrinktubing or a metal outer sheath 21 is secured over the RTD wireconnections to the stranded lead wires as shown in the drawing.

Strands of sensor wires 11 are formed as a bundle as shown in FIG. 2.The sensor wire segments all have an insulative coating as shown. Thatis true of all embodiments of this invention. For drawing clarity, thisinsulation is not specifically depicted in other figures. These wiresare bonded together with suitable material 18 such as rubber, ceramic,plastic, an appropriate potting compound, varnish or the like, to formthe otherwise unsupported, semi-rigid, elongated sensor. The bundle ispreferably round, as shown in FIG. 2, but it may take on other formssuch as square, triangular, rectangular, hexagonal or other desiredcross sections.

The configuration of FIGS. 3 and 4 is similar except that in addition tothe sensor wire 22 there are strands of pre-insulated heater wire 23.The sensor and heater wire strands would be closely adjacent or randomlyintermingled in actual practice but are shown separated in FIG. 3 toavoid confusion. This preferred configuration requires that there befour connectors to lead wires 24, 25, 26 and 27, which connections wouldbe treated in the same way as are the connector ends of the embodimentof FIG. 1. That is, each connection between the stranded lead wire andthe end of the heater or sensor wire is protected by a small piece ofshrink tubing and then the whole combination of four connections wouldnormally be encased in a short length of shrink tubing or a metal outersheath. It is preferred that the heater wires be intermingled with theRTD wires as depicted by the FIG. 4 cross section. However, they may bearranged in any relationship which would effectuate the desired result.As shown in FIG. 9, there may be common ground points for the sensor andheater. For example, lead wires 25 and 27 in FIG. 3 could be connectedtogether.

In forming the sensor as shown in any of FIGS. 1-4, the pre-insulatedsensor or sensor and heater wires are formed in a bundle,pre-impregnated with the bonding material and pulled through a heatedforming dye to compact, de-aerate and form the finished semi-rigidbundle.

With respect to FIG. 5, multiple strands of pre-insulated sensor wire 31are formed as in FIG. 1 except that they are not bonded together. Ends32 and 33 are connected to respective stranded lead wires 34 and 35 bywelding or soldering and they are enclosed by small shrink tubingelements 36 and 37 respectively. In this embodiment, the entirecombination of sensor wires and connections to the lead wires is encasedin a small diameter thin wall metal tube 41 to provide the necessarydegree of rigidity and self support. The end of the tube opposite to thelead wires is closed by means of seal plug 42 formed of suitablematerial such as metal or plastic which the tubing securely engagesafter it has been drawn down to minimum size. The seal plug is shownoversized in FIG. 5 to depict closure of the end of the tube. In thepreferred embodiment it would actually be smaller to close the end ofthe reduced diameter tube. Likewise, end 43 of the protective sheath ortubing securely engages the stranded lead wires 34, 35 and theconnections encased in shrink tubing 36 and 37.

FIG. 6 is an embodiment much like that shown in FIG. 3 with shrinktubing insulator sleeve 66 encasing the sensor and heater wires asdiscussed with respect to FIG. 5. Although shown in spacedconfiguration, pre-insulated sensor wire 45 is preferably intermingledwith heater wire 46. The ends of the respective wires 55, 56 and 57, 58are connected to lead wires 51, 52 and 53, 54 respectively, theconnections being encased in small shrink tubing elements 61, 62, 63 and64. Seal plug 65 is provided at the end opposite the connectors andshrink tubing 66 closes around the seal plug and around the connectorends as described previously.

FIGS. 5 and 6 disclose the alternatives of metal or plastic shrinktubing surrounding the sensor and heater wire strands. Plastic shrinktubing is normally activated by the application of heat. The metaltubing of FIG. 5 can be forged to the desired small diameter by drawingit through dies or by hammering it in forming dies. This action willcompact the sensor wires or the sensor and heater wire strands in muchthe same way as heating the plastic shrink tubing does. Alternatively,the voids in the bundle of wire strands can be filled by forcingsuitable potting compound in one end of the tube before plug 42 is putinto place. This has the effect of providing uniform heat transfer fromthe wire bundle to the sheath and thence to the environment. Acombination of potting and shrinking can also be employed. Finally,metal shrink tubing is also available for such uses. It functions inmuch the same way as does plastic shrink tubing in compressing thesensor and heater wire bundle for superior mechanical, thermal andelectrical properties. Note that the metal tube can also be used as aheater or even as the RTD. If used as a heater, the RTD would be wirebundle 31. If the metal tube is used as the RTD, bundle 31 would be theheater. Suitable connections would need to be made to tube 41 in eithercase.

The metal tubing can be coated or plated to promote better chemicalresistance. Not shown is a combination of shrink tubing and thin walledmetal sheaths over the shrink tubing.

An alternative embodiment is shown in FIGS. 7 and 8. In this instance,either the pre-insulated heater wire or the pre-insulated sensor wire isin the elongated form discussed with respect to FIGS. 1-6 while theother of either the sensor wire or heater wire is coiled around theelongated strands of the wire in a helical configuration. Assuming forpurposes of illustration that the elongated wire is heater wire 71 andthat the coiled pre-insulated wire is sensor wire 72. The endconnections would be sheathed in the same manner as has previously beendiscussed. The effective length of the RTD filament is greatly increasedfor increase of sensitivity and resolution by arranging the RTD filamentin this long, thin, helical form. The helical form for either the sensoror the heater also adds the advantage of adjusting the amount of wire ineach linear pass. For example, if for electrical reasons it is necessaryto get resistance equal to 21/2 linear passes, the helical winding couldbe adjusted so that 11/2 times the resistance of a single linear passcould be arranged for in the helical portion plus one straight pass ofwire in the return lead. The entire sensor configuration of FIG. 8, asshown in FIG. 7, may be enclosed in shrink tubing or in thin wall metaltubing or it may be merely bonded together and left in the form shown inFIGS. 7 and 8, as is true of the embodiments of Figs. 1-4.

Although the helical coil form of a distributed RTD has been shown inFIGS. 7 and 8 and alternatively described utilizing a heater wire core71 with no heater filaments in the coiled linear distributed RTD 72, itis to be understood that the heater wire core may be omitted andalternatively one of the self heated filament types of lineardistributed RTD's of the invention employed as the coiled element. Orthe coiled element could comprise both RTD and heater wires. There maybe a small center core of electrically inert, flexible material and theRTD wire or RTD and heater wires may be spirally wound onto that centercore. This end result can either be secured together by a bondingmaterial as discussed with respect to FIGS. 1-4 or it can be protectedby plastic or metal sheathing as discussed with respect to theembodiments of FIGS. 5 and 6.

RTD filaments of the invention may be energized by either a constantvoltage source, a constant current source, or a constant power source,each of which are known to those skilled in the art. With a constantvoltage source applied across the RTD filaments, the detection circuitrywill be arranged to detect decreases in current through the RTDfilaments resulting from increases in temperature sensed by thedistributed RTD's of the invention, and conversely will detect increasesin current resulting from decreases in temperature. With a constantcurrent source applied through the RTD filaments, the detectioncircuitry will be arranged to detect increases in voltage across the RTDfilaments resulting from increases in temperature sensed by distributedRTD's of the invention, and conversely will detect decreases in voltageresulting from decreases in temperature. For forms of the inventionshown in FIGS. 1-8, such current and voltage responses for therespective constant voltage, constant power and constant current sourcecircuits will be smooth, continuous and linear. In an alternative, butless preferred form, a constant temperature difference between theheated and unheated sensor can be arranged. The power, voltage orcurrent necessary to hold a constant temperature difference is a measureof the media flow rate or liquid level in a liquid engaging system, suchas in a tank.

An example of the electrical circuit to which the sensor of FIGS. 1-8can be connected is shown in FIG. 9. This is a simplified constantcurrent type detection circuit for use with matched pairs of distributedRTD's configured, for example, in accordance with any of FIGS. 1-8. Thereference numeral 81 generally refers to a distributed RTD of the typediscussed above, including RTD or sensor filament 82 and heater filament83 which are thermally coupled as previously discussed. The unheated orreference distributed RTD is referred to by reference numeral 84comprising RTD filament 85. The heater filament of the unheateddistributed RTD sensor is not shown in the diagram because it is notelectrically connected to the heater power source so it is not a factorin the electronic circuit of FIG. 9. It should be understood that thereference sensor would normally include a heater filament even though itis not connected so that distributed RTD 84 is, for reference purposes,an exact thermal, electrical and mechanical counterpart of heateddistributed RTD 81. Heater filament 83, one end of which is grounded, iselectrically energized through conductor 86 by electrical power source87 which may be either a constant current source, a constant voltagesource or a constant power source and is connected to a power sourcerepresented by battery 88. Alternatively, the heater may be connected toa variable power source and varied so that a constant temperaturedifference is maintained in the RTD. The amount of power required is afunction of the cooling and, for example, could be related to the amountof flow being experienced in the duct of FIG. 11 or the amount of fluidin the tank in FIG. 10.

The ungrounded ends of RTD sensor filaments 82 and 85 are connectedthrough respective conductors 91 and 92 to a pair of balanced precisionconstant current sources 93 and 94 which are both in turn connected to apower source represented as battery 95. The outputs of RTD filaments 82and 85 are connected through respective conductors 96 and 97 to theinputs of instrumentation amplifier 101. This amplifier is preferably aprecision differential amplifier having its output 102 connected toinput 103 of signal processor 104. The signal processor may be anyconventional microprocessor with adequate related performance. Theoutput of signal processor 104 is connected to output circuits 105 and106 which drive suitable instrumentation. The instrumentation is notpertinent to this invention and therefore is not shown here. However,the instrumentation may provide means to visually indicate the conditioncalculated from the outputs of the RTD sensors or it may provide controlsignals to cause something to happen such as turning devices on or off,changing liquid level or a host of other possibilities.

It should be realized that conductors 91 and 92 may be up to severalhundred feet long. Conductors 96 and 97 may be connected at any placealong the length of conductors 91 and 92 but are preferably connectednear sensor filaments 82 and 85 as shown.

Distributed RTD's according to the invention may be made as long asrequired or be other than straight, as discussed below, for spanning anyparticular field or zone, and if desired could be as long as severalhundred feet. The cross-sectional dimension of distributed RTD'saccording to the invention, on the other hand, may be extremely small,as for example having an external diameter in the range of fromapproximately 0.010 inch (0.254 mm) to approximately 0.030 inch (0.762mm). Distributed RTD's according to the invention will generally beprovided with a length that is greater than approximately 100 times itsdiameter, and in most instances it will be provided with a length thatis hundreds, or even thousands, of times greater than its diameter.

The forms of distributed RTD's shown in FIGS. 1, 2 and 5 which do notinclude a heater filament in association with the RTD wire is anaccurate linear thermometer particularly adapted to measure the averagetemperature of a nonhomogenous or nonisothermal temperature field orzone. Examples of such fields or zones are a tank of liquid havingstratified temperatures, and a flow pipe or duct with nonuniformtemperature media flowing therethrough. One or more of the distributedRTD's may be deployed across such a nonisothermal temperature field tosense the average temperature of the field. In a situation such as astratified tank of liquid where the stratification is normally in thehorizontal direction only, a single one of the distributed RTD's may beutilized in a vertically oriented deployment, or a plurality of parallelvertically deployed distributed RTD's may be employed. In a situationwhere the isothermal temperature field is more complex in cross-section,not being regularly or predictably stratified, such as nonuniformtemperature media flowing through a duct, a crossed matrix of two ormore of the distributed RTD's may be employed. Other nonuniformdistribution of the RTD or the RTD/heater combination as shown in Pat.No. 4,977,385 may also be used.

FIGS. 3, 4 and 6-8 illustrate heater-type distributed RTD's according tothe invention which include parallel multi-strand filaments, one ofwhich is an RTD wire filament and the other of which is a heater wirefilament. Distributed RTD's of the invention which include a heater wirefilament are adapted for making several different types of measurements.Perhaps the most widely useful deployment of heater-type distributedRTD's of the invention is to provide continuous gauging of the level orlocation of a phase change interface. This is typically the liquid levelor interface between liquid and gas, but may also be a liquid-to-liquidinterface between nonmiscible liquids, or the level of particulatematter in either liquid or gas. Another use of heater-type distributedRTD's of the invention is to measure the average mass flow velocity ofgas flow in large ducts where a nonuniform flow velocity distribution iscaused by flow disturbances from the presence of such fittings as tees,elbows, transitions, bends, section changes, louvers, dampers, fans,blowers and the like. Average mass flow velocity of any fluid, whethergas or liquid, in any conduit can be measured by the heater-typedistributed RTD's of the invention. The manner in which theseheater-type distributed RTD's are utilized to gauge liquid level andmeasure average flow velocity in a duct will be described in detailhereinafter. Other related information is provided in Pat. No.4,977,385.

Heater-type distributed RTD's of the invention may also be used forsensing the average temperature of a field or zone in the same manner asdescribed above for the nonheater form of the invention by simply notelectrically connecting the heater element or elements to a source ofcurrent or voltage. In the preferred form of the invention an unheateddistributed RTD would be used in conjunction with a heated distributedRTD. The unheated RTD is normally referred to as the reference RTD. In aconstant temperature field, no reference RTD would be required. In auniform (non-stratified) varying field a point RTD could be used.

The sensors of this invention will normally be employed in pairs, onebeing an unheated reference RTD while the other is the heated sensingRTD. Distributed RTD's constructed in accordance with this invention maybe used in differentially heated (gaseous or liquid) fluid flow sensors,meters and the like, or may be used for liquid level sensing, all foruse in relatively low temperature, non-hostile applications. Withrespect to the embodiment of FIGS. 1 and 2, where the RTD sensor isself-heated by forcing a low level current through it, two such sensorswould be used as indicated by the circuit of FIG. 9, creating anelectronic comparator circuit that, for example, maintains a fixedtemperature difference between the self heated and the unheatedreference RTD. When used for either determining flow rate of the mediabeing sensed or gauging liquid level in a storage tank, the qualitybeing measured by the RTD sensors measures the power required tomaintain the differential temperature as a constant.

The sensors of the other embodiments, FIGS. 3-8, would all be used inthe same way, that is, in pairs, one of them being heated and the otherproviding a reference signal output. In the embodiments having separateheater wires, only one of them is connected to a power source at onetime and the heater wire in the unheated or reference sensor is notenergized unless the first heater wire fails. In that case the sensorsmay be interchanged and the former reference sensor with the intactheater wire can be connected to the terminal formerly connected to theheater sensor and the heater. Thereafter the formerly active sensor withthe now inoperative heater wire is connected to the terminals previouslyused by the reference sensor. The function of the unconnected heaterwire is to ballast the reference sensor with the same amount of thermalmass as has the heated sensor so that even though the two sensors are atdifferent temperatures they will maintain the same temperaturedifference when transient media temperature changes occur. This is animportant feature in maintaining accurate indication in flow or fluidlevel in a tank during periods of rapid change in the sensed mediatemperature.

It is possible to employ a sensor of the type shown in FIG. 1 or FIG. 5as a mate to a sensor having separate heating wires as shown FIG. 3 orFIG. 6. It would be somewhat less expensive to manufacture such a devicebut the rate of heating and cooling of the two otherwise identical RTDsensors would be different (absent the heater wire(s)) in the presenceof changing media temperatures. Therefore, under present circumstances,it is preferred to use two identical sensors connected to the sensingcircuitry of FIG. 9.

Referring to FIG. 10, distributed RTD's of the heater types according tothe invention are deployed in matched parallel, vertical pairs forliquid level sensing. These are preferably of the types shown in FIGS.3, 4 and 6-8. FIG. 10 shows a simplified embodiment for a tank ofuniform cross section. For tanks of non-uniform cross section, extralengths of the distributed RTD may be deployed proportional to the crosssectional area of the tank. Other examples of liquid level sensingdistributed RTD's are described in Pat. No. 4,977,385.

The RTD's in the matched pair of distributed RTD's are designated 111and 112. For maximum sensitivity and resolution, they could be thecoiled version of FIGS. 7 and 8. The pair of distributed RTD's 111 and112 is deployed vertically in closely spaced, parallel relationshipinside tank 113 having side wall 114, bottom wall 115, and top wall 116.A series of mounting brackets 117 supports the two distributed RTD'swithin the tank spaced inwardly from mounting wall 114. Upper endportions 121 and 122 of RTD's 111 and 112 respectively extend upwardlybeyond top wall 116 through a panel or plug member 123 set in the topwall. This makes the stranded lead wires 124 and 125 of the two RTD's111 and 112 accessible above top wall 116. Lower ends 126 and 127 of theRTD's may be disposed as closely as desired to bottom tank wall 115.Reference is again made to Pat. No. 4,977,385.

To operate the pair of distributed RTD's for gauging liquid level intank 113, the heater filament of one of the RTD's is energized, whilethe heater filament of the other RTD is left unenergized. Assuming thereis quiescent liquid in the tank up to a level such as that designated byphantom horizontal line 131, and still air in the rest of the tank abovethe liquid, the relatively high density, high heat transfer liquid willdisperse heat away from the wetted portion of the heated distributed RTDmuch more efficiently than the relatively poor thermal dispersion rateof air for the unwetted portion. Preferably, sufficient current orvoltage is supplied to the heater filament of the heated RTD sensor toheat the portion of it that is in air to a differential temperature of,for example, approximately 100° F. above the temperature of the unheatedRTD sensor which serves as a reference. The much more efficient thermalconductivity of the quiescent liquid will cool the immersed portion ofthe heated RTD to a differential temperature on the order of only about10° F. above the unheated reference RTD. As the liquid levelprogressively rises in tank 113, a progressively greater length of theheated RTD is cooled off to the approximately 10° F. temperaturedifferential and a progressively shorter length of the heated RTD willstill have the high, approximately 100° F. temperature differential.Conversely, as the liquid level drops in the tank, a progressivelygreater length of the heated RTD will be uncovered and therefore subjectto the high temperature differential of approximately 100° F. At thesame time a progressively shorter length of the heated RTD below liquidlevel 131 will be cooled to the low, approximately 10 F., temperaturedifferential.

The resulting continuous variation of electrical resistance in theheated RTD relative to the electrical resistance in the unheated RTDrelating to variations in liquid level in the tank will permit goodaccuracy, high resolution, smooth, continuous, linear, nonsteppedgauging and analog reading of liquid level in the detection circuitryand instrumentation electrically connected to lead wires 124 and 125 ofthe two continuous RTD's. Such high accuracy, resolution and continuousgauging not only enable the exact liquid level to be determined at anytime, but also enable liquid level changes to be immediately sensed, ascompared with prior art point sensing systems which could allow aconsiderable liquid level change to occur without detection. This alsoavoids the large time and economic costs associated with the manyconnections required to be made when multiple individual point sensorsare employed.

In the event the liquid and/or air in the gauged vessel may be turbulentrather than quiescent, then greater cooling of heated distributed RTD111 or 112 will be caused by the turbulence, and the aforementioned 10°F. and/or 100° F. temperature differentials will be correspondinglydecreased. If the turbulence is constant, the system can be calibratedin an equally turbulent environment, and provide readouts ofsatisfactory accuracy. On the other hand, if the turbulence is randomlyvarying such that variable values of cooling occur, the system canproduce errors in the readout caused by the random turbulence. If suchis the case, then a still well or stilling well can be provided whichsurrounds at least the heated distributed RTD, and preferably both theheated and unheated RTD's so that it can also serve as a support forboth, to mitigate the effects of the varying turbulence in the liquidand the gas phases. In tanks such as spherical ones or cylindrical tanksmounted on their sides, RTD's with non-uniform distribution may beemployed to take into account the large volume of liquid per unit ofheight that occurs near the vertical center of the tank as opposed tothe lesser volume of liquid per unit of height near the top and bottomof the tank. Once again, examples are given in Pat. No. 4,977,385 foranother distributed RTD.

FIGS. 11 and 12 illustrate use of a matched pair of distributed RTD'saccording to the invention to measure the average mass flow in a ductwhere a flow of nonuniform distribution exists which may be created, forexample, in air ducts where diameters are large and fittings such astees, elbows, transitions, bends, section changes, louvers, dampers, andthe like cause flow disturbances. Another example of a large duct whereit would be desirable to measure the average mass flow velocity or rateis the input air duct to or the combustion products output duct from afossil fuel power plant, or both. The RTD's in the matched pair ofdistributed RTD's in FIGS. 11 and 12 are designated 133 and 134, andthese are heated type RTD's as previously described. RTD's 133 and 134are shown deployed in spaced, parallel relationship to each otherdiametrically across air duct elbow 135 in the direction from thesmallest radius of curvature at inside 136 of the duct bend to thelargest radius of curvature at outside 137 of the duct bend. Thisorientation is chosen because the RTD's are arranged in the direction ofgreatest nonuniformity of flow distribution but symmetrical in thenormal axis. Other symmetrical positions in the duct are possible andmay be preferred as the case-by-case conditions may indicate. Thestructure shown in FIG. 12 is a simplified embodiment. In the preferredform two RTD's would be arranged so that the length of the sensing RTDmaterial at any location would be proportional to the area of the ductbeing interrogated. Again, examples are disclosed in Pat. No. 4,977,385with a different distributed RTD.

Front end portions 141 of the distributed RTD's which carry the leadwires extend externally of the wall of the duct through a suitablesupport body or connector plug 142 in the region of the outer curvature137 of the elbow, with lead wires 143 accessible for connection to thedetection circuitry and current or voltage source. Rear end portions 144of the RTD's are secured by a suitable internal support body 145 in theregion 136 of the inner curvature of the elbow.

The heater filaments of one of distributed RTD's 133 or 134 areenergized, and the heater elements of the other are not, the unheatedRTD serving as a reference. The flowing air cools the heated RTD sensoras a function of the mass flow rate passing the heated RTD. Relativelyhigh flow rates near the longer outer curvature region 137 of the elbowwill cool that portion of the heated RTD to a greater degree than thecooling that will occur near the shorter inner curvature region 136 ofthe elbow. The resulting sensed output of RTD filaments in the heateddistributed RTD as compared to the sensed output of the unheated RTDfilaments will provide an excellent summation and average of the massflow rate passing the matched pair of distributed RTD's. Naturally, itis necessary to distribute the RTD's so that no one area of the ductcross section is unduly influenced.

The uniform distribution for the RTD as shown in FIG. 12 would be mostappropriate if the duct were rectangular, as shown by dotted lines 151.Several examples are given in application Pat. No. 4,977,385 for optimumdistribution of the sensor in non-rectangular ducts. The configurationof the RTD shown in FIG. 13 is one method of giving proper influence tovariations in duct elbow cross section. Duct 152 is fitted with theheated RTD 153 and reference RTD 154. Heater RTD 153 is shown in avarying sinuous configuration. The extra length of the heated RTD ispositioned in the center of the duct because more area at that locationrepresents more potential flow. The lateral length L of the loops isproportional to the area H_(A) between the dotted lines. Similar spacingfrom the center outward results in less area and therefore shorterlateral sensor loops. Thus the influence of greater potential flow areais recognized and compensated for by locating extra heated sensor lengthin the central portion than in the less influential outer reaches.

The above is true only when the distributed RTD sensor arrangement islocated in an elbow. If it were placed in a long straight duct it wouldbe appropriate to form the RTD arrangement shown in FIG. 14. Here moreRTD length 155 is in the outer areas closer to the duct walls 156 thanin the center. That is because, for example, one inch of radius at theinner periphery of the duct (A₂) represents more potential flow volumethan one inch at or near the center (A₁). That relationship, shown inFIG. 14, is represented by the following equation: ##EQU1## where L₁ andL₂ are the RTD lengths in respective areas A₁ and A₂. Reference RTD 157is shown in FIG. 14 as though positioned closely upstream or downstreamfrom the sinuous heated RTD.

If additional flow rate information is desired for the summing andaveraging of the flow rate in the duct elbow, one or more additionalmatched pairs of spaced, parallel heated distributed RTD units may bedeployed across the elbow, in close to the same transverse plane acrosselbow 135, but offset angularly as viewed in FIG. 12 from pair 133 and134 of RTD's. Alternatively, if desired, a gridwork of crossing matchedpairs of distributed RTD's might be deployed across the duct elbow.

Should the mass flow rate be so high as to cool heated RTD 133 or 134 tosuch a degree as to saturate its signal output, that is, where increasedflow rate causes little or no additional cooling, a shroud can beprovided to increase the usable range. The shroud proportionally reducesthe rate at which heat is dispersed from the heated distributed RTD intothe flowing stream.

Utilization of a matched pair of heater-type distributed RTD's of theinvention in each of the liquid level gauging and mass flow rate sensingserves two purposes. First, the unheated distributed RTD, by havingidentical components as the heated distributed RTD, has an identicalthermal response to the environment other than that response which isthe result of energizing the heater, so that a true differentialtemperature response is provided by comparison of the output of the RTDfilaments of the heated RTD relative to the output of the RTD filamentsof the unheated RTD. Second, as discussed above, with the two matchingdistributed RTD's, if the heater should fail in one of them, then theheater of the other one may alternatively be energized. In this manner,only the external electrical connections need be changed, which can bedone at the location of the detection circuitry. Then the formerlyheated RTD sensor will serve as the reference sensor RTD sensor, and thedifficulty and expense of replacing the damaged RTD sensor is avoided.

One of the significant advantages of this invention is low cost ofconstruction, at least partially related to the simplicity of formingthe sensor. The FIG. 2 embodiment can be made by winding insulated wireback and forth between two pegs arranged so that an RTD sensor of thedesired length will be made. The number of lengths or passes and thegauge of the wire will determine the nominal or room temperature directcurrent resistance. The stranded lead wires can be attached by welding,soldering or other common means and short pieces of shrink tubing areplaced over the bare conductors. The short tubes are then heated andfixed in place. The heater wire is prepared much the same as the RTDwire and may be done simultaneously to facilitate intermingling. Ofcourse, it is not necessary that the wires be intermingled in thesensor. Preferably a slow setting rubbery sealant is coated onto thestranded wires of the FIG. 2 embodiment and the wires are pulled intothe shrink tubing. Alternatively, potting compound such as rubber, epoxyor other flexible material can be forced under pressure into one end ofthe shrink tubing prior to shrinking the outer sheath. The seal plug isplaced in the open end of the tubing and heat is then applied to shrinkit down onto the sensor and heater wire, connectors and the seal plug.Any surplus of the rubbery sealant is forced out of the open end of theshrink tube and disposed of. The wire may be conventional ML insulatedcopper wire. Such wire has been insulated with varnish in the past but adispersion coating of a polyimide provides a coating more resistant toabrasion and heat.

In this manner, the heated RTD sensor is formed which can bestrategically located within a tank, pipe or duct and used to detectliquid level or flow rate as previously described.

Alternatively, the shrink tubing of FIG. 6 can be replaced with a metaltube as shown in FIG. 5. This tube may be drawn down in successivelysmaller dyes until the wires are firmly packed inside the metal sheath.Other methods of reducing the metal tube diameter have been discussed.The seal plug can be welded in place in either the metal sheathed caseor captured in the shrink tube assembly to form a hermetically sealedend for exposure to the media.

In another version of the sensing system, the sensor instrument in FIG.1 may be employed alone in either a storage tank to determine liquidlevel or in an irregular duct where an irregular or variable flowpattern or both occurs. In this configuration, it is assumed that smalltemperature changes occur in the process media and therefore a second orreference sensor need not be employed. In this case, a fixed amount ofheater power would be impressed on the sensor wire and the flow rate orliquid level can be determined by relating the cooling experienced tothe amount of cooling caused by predetermination of the media conditionunder known conditions.

Additionally, it is possible that the instrument of either of theembodiments discussed above can be combined with any other type oftemperature sensors such as a thermocouple, a customary point sensingRTD, temperature transmitters or temperature sensing strain gauges toname just a few. This arrangement would be most economical if one pointin the media is equal to or consistently representative of the mediatemperature throughout the tank, duct, boiler or any manner of vessel orweir. If the reference temperature is being employed to satisfy the userby any of the point sensing means mentioned above, the distributed RTDcan be heated by a fixed amount and the differential temperaturemeasured or a constant differential temperature and variable heaterpower applied and measured for the determination of the media condition.

As an alternative to using point type sensors, a device constructed inaccordance with FIG. 1 can be formed with substantially reduced lengthsfor use with longer lengths of either the FIG. 1 or the FIG. 3embodiments. The use of sensors constructed in accordance with thisinvention but in very short lengths would be equivalent to the pointsensors mentioned above and would be used to sense the mediatemperature.

A further alternative embodiment would employ braided sheathingmaterial, either metal or plastic, in place of the shrink tubing of FIG.6 or the thin wall metal tubing of FIG. 5. Metal sheathing either in thebraided or tube version would likely be employed to electrically shieldthe sensor from magnetic or electric field effects.

In view of the above description it is likely that improvements andmodifications will occur to those skilled in the art which are withinthe scope of the appended claims.

What is claimed is:
 1. A method for forming a distributed resistancetemperature sensor comprising elongated resistance temperature detector(RTD) wire, said method comprising the steps of:forming an insulativecoating on said RTD wire; forming said insulated RTD wire into a single,continuous, unitary, longitudinal configuration of multiple elements,each of predetermined length; securing said insulated RTD wire elementstogether in a physically parallel, electrically series configuration toform a unitary, semi-rigid, self supporting structure about one elementlong; and mounting first connecting means to the ends of said RTD wire,the connecting means being adapted to be connected to detector circuitryand to an external source of electrical power.
 2. The method recited inclaim 1, wherein said securing step comprises forming a protectivesheath closely around the connecting means.
 3. The method recited inclaim 2, wherein said protective sheath is a thin metal tube.
 4. Themethod recited in claim 2, wherein said protective sheath is plasticshrink tubing.
 5. The method recited in claim 1, wherein said sensorincludes means for heating said RTD wire, said heating means comprisinga heater wire, said method comprising the further steps of:forming aninsulative coating on said heater wire; forming said insulated heaterwire into a longitudinal configuration of multiple elements in closeproximity to said RTD wire elements; and mounting second connectingmeans to the ends of said heater wire, said second connecting meansbeing adapted to be connected to an external source of electrical power.6. The method recited in claim 5, wherein:said heater wire forming stepforms an elongated configuration of a plurality of parallelinterconnected elements; and said securing step comprises applyingbonding material to said RTD and heater wire elements to secure themtogether.
 7. The method recited in claim 6, and comprising the furtherstep of interleaving said heater wire strands with said RTD wirestrands.
 8. The method recited in claim 6, wherein said securing step ofapplying bonded material comprises forming a protective sheath closelyaround said combined longitudinal configuration of multiple RTD elementsand heater wire.
 9. The method recited in claim 5, wherein:said heaterwire forming step forms an elongated configuration of a plurality ofparallel interconnected elements; and said securing step comprisesforming a protective sheath closely around the first and secondconnecting means.
 10. The method recited in claim 9, wherein saidprotective sheath is formed from thin metal tube.
 11. The method recitedin claim 9, wherein said protective sheath is formed from plastic shrinktubing.
 12. The method recited in claim 1, wherein:said securing stepcomprises applying bonding material to said RTD elements to secure themtogether.
 13. The method recited in claim 12, wherein said securing stepof applying bonded material comprises forming a protective sheath aroundsaid longitudinal configuration of multiple RTD elements.
 14. The methodrecited in claim 5, wherein:one of said RTD and heater wire formingsteps forms an elongated configuration of a plurality of elongatedparallel strands; and then the other of said RTD and heater wire formingsteps forms the wire in a helical fashion over the elongated parallelstrands.
 15. The method recited in claim 1, wherein said sensor is selfheated so that its quiescent condition is at a temperature elevated fromambient temperature and sensing of change results from said sensor beingat least partially immersed in a fluid which reduces the temperature ofthe immersed portion, and thereby changes the resistance of said RTDwire.
 16. A method for forming a distributed resistance temperaturesensor from elongated, insulated resistance temperature detector (RTD)wire, said method comprising the steps of:forming said insulated RTDwire into a single, continuous, unitary, longitudinal configuration ofmultiple elements, each of predetermined length; securing said insulatedRTD wire elements together in a physically parallel, electrically seriesconfiguration to form a unitary, semi-rigid, self supporting structure;and mounted first connecting means to the ends of said RTD wire, theconnecting means being adapted to be connected to detector circuitry andto an external source of electrical power.
 17. The method recited inclaim 16, wherein said sensor includes means for heating said RTD wire,said heating means comprising insulated heater wire, said methodcomprising insulated heater wire,forming said insulated heater wire intoa longitudinal configuration of multiple elements in close proximity tosaid RTD wire elements; and mounting second connecting means to the endsof said heater wire, said second connecting means being adapted to beconnected to an external source of electrical power.
 18. A method forforming a distributed resistance temperature sensor from elongatedresistance temperature sensor (RTD) wire, said method comprising thesteps of:forming an insulative coating on said RTD wire; forming saidinsulated RTD wire into a longitudinal configuration of multipleelements: securing said insulated wire RTD elements together by applyingbonding material of said RTD elements to form a unitary structure; andmounting first connecting means to the ends of said RTD wire, theconnecting means being adapted to be connected to detector circuitry andto an external source of electrical power.
 19. The method recited inclaim 18, wherein said securing step of applying bonding materialcomprises forming a protective sheath around said longitudinalconfiguration of multiple RTD elements.
 20. A method for forming adistributed resistance temperature sensor from elongated resistancetemperature detector (RTD) wire and elongated heater wire, said methodcomprising the steps of:forming an insulative coating on said RTD wire;forming an insulative coating on said heater wire; forming saidinsulated RTD wire into a longitudinal configuration of multipleelements; forming said insulated heater wire into a longitudinalconfiguration of multiple elements in close proximity to said RTD wireelements; securing said insulated RTD wire elements and said insulatedheater wire elements together to form a unitary structure; mountingfirst connecting means to the ends of said RTD wire, the connectingmeans being adapted to be connected to detector circuitry; and mountingsecond connecting means to the ends of said heater wire, said secondconnecting means being adapted to be connected to an external source ofelectrical power; wherein said heater wire forming step forms anelongated configuration of a plurality of parallel interconnectedelements; and said securing step comprises applying bonded material tosaid RTD and heater wire elements to secure them together.
 21. A methodfor forming a distributed resistance temperature sensor from elongatedresistance temperature detector (RTD) wire and heater wire, said methodcomprising the steps of:forming an insulative coating on said RTD wire;forming an insulative coating on said heater wire; forming saidinsulated RTD wire into a longitudinal configuration of multipleelements; forming said insulated heater wire into a longitudinalconfiguration of multiple elements in close proximity to said RTD wireelements; securing said insulated RTD wire elements and said insulatedheater wire elements together to form a unitary structure; mountingfirst connecting means to the ends of said RTD wire, the connectingmeans being adapted to be connected to detector circuitry; and mountingsecond connecting means to the ends of said heater wire, said secondconnecting means being adapted to be connected to an external source ofelectrical power; said heater wire forming step forms an elongatedconfiguration of a plurality of parallel interconnected elements andsaid securing step comprises forming a protective sheath closely aroundthe first and second connecting means.
 22. A method for forming adistributed resistance temperature sensor comprising elongatedresistance temperature detector (RTD) wire and heater wire, said methodcomprising the steps of:forming an insulative coating on said RTD wire;forming an insulative coating on said heater wire; forming saidinsulative RTD wire into a single, continuous, unitary, longitudinalconfiguration of multiple elements, each of predetermined length;forming said insulated heater wire into a longitudinal configuration ofmultiple elements in close proximity to said RTD wire elements; securingsaid insulated RTD wire elements and said insulated heater wire elementstogether by means of a protective sheath around said RTD wire and heaterwire combined to form a unitary structure, said RTD wire elements beingarranged in a physically parallel, electrically series configuration;mounting first connecting means to the ends of said RTD wire, theconnecting means being adapted to be connected to detector circuitry;and mounting second connecting means to the ends of said heater wire,said second connecting means being adapted to be connected to anexternal source of electrical power.